To meet current emission regulations, automotive vehicles must regulate the air/fuel ratio (A/F) supplied to the vehicles' cylinders so as to achieve maximum efficiency of the vehicles' catalysts. For this purpose, it is known to control the air/fuel ratio of internal combustion engines using an exhaust gas oxygen (EGO) sensor positioned in the exhaust stream from the engine. The EGO sensor provides feedback data to an electronic controller that calculates preferred A/F values over time to achieve optimum efficiency of a catalyst in the exhaust system. It is also known to have systems with two EGO sensors in the exhaust stream in an effort to achieve more precise A/F control with respect to the catalyst window. Normally, a pre-catalyst EGO sensor is positioned upstream of the catalyst and a post-catalyst EGO sensor is positioned downstream of the catalyst. Finally, in connection with engines having two groups of cylinders, it is known to have a two-bank exhaust system coupled thereto where each exhaust bank has a catalyst as well as pre-catalyst and post-catalyst EGO sensors. Each of the exhaust banks corresponds to a group of cylinders in the engine. The feedback signals received from the EGO sensors are used to calculate the desired A/F values in their respective group of cylinders at any given time. The controller uses these desired A/F values to control the amount of liquid fuel that is injected into the cylinders by the vehicle's fuel injector. It is a known methodology to use the EGO sensor feedback signals to calculate desired A/F values that collectively, when viewed over time, form A/F waveforms having ramp portions, jumpback portions and hold portions, as shown in FIG. 4.
In order to build two-bank exhaust systems more economically, it is known to eliminate one of the post-catalyst EGO sensors in a two-bank, four-EGO sensor system. Specifically, it is known to eliminate the post-catalyst EGO sensor in one of the banks and move the post-catalyst EGO sensor from the other bank downstream in the system to where the exhaust gases from the two banks are combined prior to being expelled from the system. This system is known as the so-called Y-pipe system and is shown generally in FIG. 2. In the Y-pipe system, the single downstream EGO sensor performs the function of monitoring the oxygen content of the post-catalyst exhaust gases for both banks. However, because the exhaust gases from the two banks are combined prior to reaching the downstream EGO sensor, the data provided from the downstream EGO sensor is derived from the mixture of the exhaust gases from the two banks. Thus, because the downstream EGO sensor is unable to distinguish between the oxygen contents of the separate banks, the feedback data provided by the downstream EGO sensor is not specific to either bank. Accordingly, the A/F levels for the individual banks cannot be monitored and controlled as closely, and, as a result, the potential performance of the system is limited.
Therefore, it is desirable to have an improved system and methodology for controlling the A/F levels in each exhaust bank of a two-bank system using only three EGO sensors.